Resolve "dig +nssearch sometimes hangs in BIND v9.16, with an assertion failure after "ctrl+c"
Fix dig +nssearch race between recv_done() and send_done().
The send_done() callback needs to access query's link.next pointer
when running in +nssearch mode, even if the query is already canceled
or serviced, which can happen when recv_done() happens to be called
earlier than send_done().
Keep the next query's pointer before unlinking the query from the
lookup's queries list in clear_query() so that send_done() can
use it even if the query is cleared.
Closes #3278 (closed)